turbo cooling down

[Andy Leszczynski]

Hi,

is it true that turbo engines should be shut down right away but some
times is needed to cool down the turbine by running at low RPM?

A.

 

[Josh Assing]

Can you please point me to anyplace that suggests that turbo engines should be
shut down "Righ taway"?

Turbines get very very hot -- if you come in your driveway after driving hard;
the turbine will be hot.... it should be cooled donw a bit...

However, if you have an intercooler; they can probably shut down "sooner" vs
letting them cool down for a few minutes...

Hi,

is it true that turbo engines should be shut down right away but some
times is needed to cool down the turbine by running at low RPM?

A.

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[Coggo]

As a general rule turbo engines should be allowed to run for a short time if
they have been working hard, to allow the turbo to cool a little and avoid
cooking the bearings. Many modern turbos are water cooled, and don't need
any cool down period.

Dave

 

[Holly Wood]

Do you have any idea what an intercooler does, Dickhead?

 

[Josh Assing]

Do you have any idea what an intercooler does, Dickhead?

what did I say that warranted "dickhead" as response?


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[xblazinlv]

I've never heard anyone give the advice that a turbo charged vehicle
should be shut down right away, probably because it is common knowledge
that exactly the opposite should be done. Thus the reason why people
buy turbo timers...


As for the dickhead comment, that is very unnecessary.

Even though an intercooler has nothing to do with the turbo getting hot
or needing to wait before shutting it down. The intercooler delivers
cooler air temperatures to the motor after the turbo has compressed the
air, it has nothing to do with cooling the turbo.

 

[y_p_w]

what did I say that warranted "dickhead" as response?

I don't know if that response was warranted, but intercoolers
do **nothing** to cool the turbine. They cool the air going
into the engine, which allows for higher boost pressure. It
really means you're getting more air into the engine. Since
an engine is essentially an air pump, more air = more power.

The turbo bearings in a WRX (and most modern water-cooled turbos)
are cooled by both the flow of motor oil and the main cooling
system.

Many older turbos were only cooled by oil flow alone. Letting
the engine idle for some time would allow the turbo to spin down
and the oil flow to cool the heat in the bearings down to a
reasonably low temperature. If you didn't do so, it would get
even hotter as the bearings spun with only a residual amount of
oil after the oil pump was shut off. High heat concentrated on a
small amount of oil meant that a bit of the oil would break down
into carbon - AKA coking. True synthetic oils are more resistant
to this, and were often used in oil-cooled turbos to reduce the
chance of coking. The effects of coking are gradual. I'm sure
there's increased friction from hard deposits on the bearings,
and eventually it'll severely impede the flow of oil that both
lubricates and cools the bearings.

The WRX cooling system is built to reduce the chance of coking.
If you look at the cooling system, there's a small coolant
tank (with a cap) located higher than the engine. When the
engine is shut off, this coolant in this tank will invariably be
cooler than the turbo temps. The heat from the turbo will be
drawn through the coolant to this tank. This works even though
the oil and water pumps are shut off.

The flow of oil is still mostly shut off, and there will be a
residual amount of oil. However - keeping the temperature
down greatly reduces the chance of coking. I still let my
WRX idle for 30 sec to a minute after climbing a hill, but
it's probably overkill.

 

[Phil]

I know with earthmoving equipment the turbo is usually slowed down first
with the engine idling to allow it to get cooled and lubricated while coming
to a standstill. I once had a four wheel drive with a turbo diesel which had
an idling timer that could be preset for up to ten minutes so that you could
lock and leave the vehicle with the engine running while the turbo slowed as
the engine continued running. After the preset time, the engine was shut
off. So in answer to your question I would say yes.

 

[Michael Pardee]

As a general rule turbo engines should be allowed to run for a short time
if they have been working hard, to allow the turbo to cool a little and
avoid cooking the bearings. Many modern turbos are water cooled, and don't
need any cool down period.

Dave

Our experience with our '85 turbo Volvo supports that. It was my wife's car
for a decade and I couldn't convince her not to race the engine when she
started it in the morning. I winced when she pulled off the freeway into gas
stations and shut down the engine right away - I could hear the turbo spin
down after she shut the engine off. The 20 year old car has almost 240K
miles on it now and the water cooled turbo is still hanging in there.

Mike

 

[CompUser]

The WRX cooling system is built to reduce the chance of coking.
If you look at the cooling system, there's a small coolant
tank (with a cap) located higher than the engine. When the
engine is shut off, this coolant in this tank will invariably be
cooler than the turbo temps. The heat from the turbo will be
drawn through the coolant to this tank. This works even though
the oil and water pumps are shut off.

Kinda sorta.

It's actually working on convection. The coolant
in turbine housing will heat (and possibly
steam), rising up toward the higher catch can.
As it moves, "cooler" coolant will flow up behind
it, keeping the turbine housing filled with
coolant.

 

[y_p_w]

Kinda sorta.

It's actually working on convection. The coolant
in turbine housing will heat (and possibly
steam), rising up toward the higher catch can.
As it moves, "cooler" coolant will flow up behind
it, keeping the turbine housing filled with
coolant.

It's been a while since I've thought about it. The
term I heard used to describe it is "thermal siphon".

 

[y_p_w]

Our experience with our '85 turbo Volvo supports that. It was my wife's car
for a decade and I couldn't convince her not to race the engine when she
started it in the morning. I winced when she pulled off the freeway into gas
stations and shut down the engine right away - I could hear the turbo spin
down after she shut the engine off. The 20 year old car has almost 240K
miles on it now and the water cooled turbo is still hanging in there.

It's a combination of factors that made a cool-down period for
(only) oil-cooled turbos important. If it's been run hard, the
turbo housing is going to be hot (I've heard it can glow red) even
though the oil cools the bearings. Then there's energy stored in
the turbine. An immediate shut down would stop the oil flow, the
heat from the turbo housing would transfer to the bearings without
being cooled, and the energy from the turbo would create even more
heat as it spun down. All this is happening in a small pool of oil
that isn't going anywhere. The consequence was overheated bearings
and "coking" of the oil that remained. A "cool-down" allows the
turbo housing to cool and the turbine to slow down such that the
heat sources before shut-down are severely reduced.

Adding water cooling doesn't slow down the turbine, doesn't cool
down the turbo housing, and doesn't restore the flow of oil.
However - it will draw the heat away that gets to the bearings
from all these sources. I still allow for a small cool-down
period, but I don't obsess over it. I actually back down an
incline to my garage in neutral. The time it takes is from 20-
40 seconds.

 

[k. ote]

I don't know if that response was warranted, but intercoolers
do **nothing** to cool the turbine. They cool the air going

.... except that the cooler air is flowing through the turbine, which is
forcing the air into the engine. Since this heats the air, conservation of
energy that states that that heat is drawn *from* the turbine. Heat doesn't
appear out of nowhere. Thus, the intercooler does have a (probably
mechanically insignificant) effect on cooling the turbine.

 

[CompUser]

... except that the cooler air is flowing through the turbine, which is
forcing the air into the engine. Since this heats the air, conservation of
energy that states that that heat is drawn *from* the turbine. Heat doesn't
appear out of nowhere. Thus, the intercooler does have a (probably
mechanically insignificant) effect on cooling the turbine.

We're rapidly approaching the point of how many
angels can dance on the head of a pin...;-)

A couple of points:

The air passing thru the intercooler is coming
from the turbocharger *compressor*. The
turbocharger turbine is on the other side of the
house, handling exhaust gasses only. AFA as the
IC having a cooling effect on the compressor,
ordinarily it wouldn't, since it's downstream of
the compressor's output. The only time the
compressor would benefit from the IC's cooling
effect would be during bypass valve operation.


Heat doesn't appear out of nowhere, it appears as
a function of compression of the air
passed by the compressor, according to the ideal
gas law:

PV=nRT

With V (volume of the system), n (amount of gas
IIRC) and R (a constant--Avogadro's Number? High
school was a _long_ time ago!) remaining
unchanged, if we boost pressure (P), the only
"flex" on the other side of the equation is T,
temperature.

 

[BD]

I still let my WRX idle for 30 sec to a minute after climbing a hill, but it's probably overkill.

Question: is it not also sufficient to ensure that you keep the revs
below 3000 rpm for a couple of minutes before stopping the vehicle? I
have an 02 WRX and my understanding is that the turbo does not kick in
below 3000, so if you're (for example) winding gently around city
streets at 2500 rpm before rolling into the driveway, it's okay to just
shut the engine off at that point...

Sure hope so, because that's always been my practice...

BD